The present invention relates to an improved catalyst activation process for olefinic naphtha hydrodesulfurization and using the activated catalyst to desulfurize a catalytically cracked naphtha. The activation process maintains the sulfur removal activity of the catalyst while reducing the olefin saturation activity during hydrodesulfurization.
Hydroprocessing petroleum feedstocks to remove heteroatoms, particularly sulfur, is critical to modem refiners in order to meet ever more demanding product quality specifications. Hydroprocessing to remove heteroatoms from hydrocarbon is often referred to as hydrotreating, and hydrodesulfurization refers to a hydrotreating processes for sulfur removal. In hydrodesulfurization, feed sulfur is removed by conversion to hydrogen sulfide, and is typically achieved by reaction with hydrogen over non-noble metal sulfide catalysts, especially those of Co/Mo and Ni/Mo, at fairly severe temperatures and pressures to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process.
Some naphtha fractions contain olefins, such as, for example, catalytically cracked naphthas or coker naphthas, which typically contain over about 20 wt. % olefins. At least a portion of the olefins are hydrogenated to saturated hydrocarbons during the hydrodesulfurization operation. Since the olefins are high octane components, for some motor fuel use, it is desirable to retain the olefins rather than to convert them to saturated compounds. Conventional fresh hydrodesulfurization catalyst have both hydrogenation and desulfurization activity. Hydrodesulfurization of cracked naphthas using conventional naphtha desulfurization catalysts under conventional startup procedures and under conditions required for sulfur removal, produces a significant loss of olefins through hydrogenation. This can result in a lower grade fuel product which may need further processing to form higher octane fuel, adding significantly to production expenses.
Selective hydrodesulfurization to remove sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as selective catalysts, have been described. For example, U.S. Pat. Nos. 4,132,632 and 4,140,626 disclose selective desulfurizaton of cracked naphthas by using specific catalysts having particular amounts of Group VI and VIII metals on magnesia support. U.S. Pat. No. 4,149,965 discloses a process for starting-up naphtha hydrodesulfurization using partially deactivated hydrotreating catalyst under relatively low pressure of less than 200 psig. The catalyst is partially deactivated using a substantially non-metals containing hydrocarbonaceous oil for a time ranging from about 10 hrs to about 20 days. U.S. Pat. No. 2,983,669 discloses a processes for treating petroleum having a high sulfur content using fractionation and hydrodesulfurization. It is suggested in this ""669 patent that the hydrodesulfurization catalyst be one that does not readily promote hydrogenation, such as a partially spent catalyst.
Hydrodesulfurization catalysts age, losing activity during use by collecting deposits of carbonaceous material, impurities, such as metals, from the treated feedstock, or both. Eventually, with increased deposition, the catalyst is no longer able to provide effective hydrodesulfurization. The deactivated catalyst may be regenerated and reused, but is generally less effective than fresh catalyst by requiring higher temperature to give the desired activity and becoming deactivated more quickly than fresh catalyst. Although hydrodesulfurization catalysts can usually be repetitively regenerated, they eventually become irreversibly deactivated, or spent, essentially losing their intended hydrodesulfurization utility.
It is known to hydrodesulfurize an olefinic naphtha by adding a nitrogen compound to the feed in order to deactivate the catalyst for the hydrodesulfurization reaction. See U.S. Pat. No. 2,913,405.
It is also known to hydrotreat a straight run fuel oil at a pressure not greater than 150 psig and at a temperature from 400xc2x0 to 500xc2x0 F. in the presence of a catalyst that had been employed in a prior hydrotreating process operated at a higher pressure than the pressure of the fuel oil hydrotreating step until the catalyst had been substantially permanently deactivated for the purpose of said high pressure process, as shown in U.S. Pat. No. 3,870,626.
Spent hydrodesulfurization catalysts have been used in hydrodesulfurization. For example, U.S. Pat. No. 3,876,532 discloses a process for hydrodesulfurizing middle distillate, virgin oils using spent hydrotreating catalysts under extremely mild conditions to reduce acid and mercaptan content, to remove sulfur below 0.2 wt. %, or 2,000 ppm. U.S. Pat. No. 4,414,102 discloses the use of spent hydrodesulfurization catalyst to transform nitrogen- or oxygen-containing compounds to sulfur-containing compounds followed by mild hydrodesulfurization treatment. Also, U.S. Pat. No. 5,286,373 discloses a process for selectively hydrodesulfurizing naphtha by contacting the naphtha, which contains olefins and thiohydrocarbons, with hydrogen under vigorous hydrodesulfurization conditions in the presence of essentially deactivated hydrodesulfurization catalyst which selectively produces hydrogen sulfide and desulfurized hydrocarbons and resulting in a relatively high olefin content.
Although some of the above processes have met with commercial success, there still remains a need in the art for improved activation process for cat naphtha desulfurization catalysts that do not require the use of an additional deactivation step.
In one embodiment, the invention relates to a hydrodesulfurization process using an activated Co/Mo supported catalyst, comprising.
(a) heating a Co/Mo supported catalyst to a first temperature of about 350xc2x0 F. to about 450xc2x0 F. in a first effective amount of time, in the presence of hydrogen and hydrogen sulfide, and in the presence of a substantially olefin-free virgin naphtha at a first effective pressure such that the environment is not a reducing environment;
(b) holding the Co/Mo supported catalyst at 350xc2x0 F. to about 450xc2x0 F. for a second effective amount of time so that at least 20% of the metals capable of sulfiding will sulfide;
(c) further heating said Co/Mo supported catalyst to a second temperature from about 550xc2x0 F. to 700xc2x0 F. in the presence of hydrogen and hydrogen sulfide and in the presence of the virgin naphtha and at a second effective pressure such that the environment is non-reducing, so that substantially all of the metals are sulfided in order to form the activated CoMo supported catalyst; and
(d) selectively hydrodesulfurizing a feedstock containing a cracked naphtha in the presence of a catalytically effective amount of the activated CoMo supported catalyst under selective hydrodesulfurization conditions.
In one embodiment the temperature of step a) is obtained by ramping at a rate of about 10xc2x0 F. to about 80xc2x0 F. per hour.
In another embodiment the total pressure is from about 200 to 400 psig.
In yet another embodiment, a selectively hydrodesulfurized product is conducted away from the process for storage or further processing, such as blending, and especially blending to make gasoline and other fuel products.